Secondary lithium batteries having an intercalation compound as cathode and a lithium metal anode have been extensively studied because of their commercial potential. The bulk of these studies have been concerned with liquid electrolyte cells, having cell voltages in the vicinity of 3.0 volts, which is readily obtainable with vanadium oxide intercalation cathodes and lithium metal anodes. Solid electrolyte cells, particularly, solid polymeric electrolyte cells, have also seen increasing interest. While the high specific capacity (about 380 AhKG.sup.-1) of a lithium cells and their high voltage provide energy densities higher than other electrochemical systems, it is believed that these cells can be improved in several ways. Specifically, we look to cathodes of higher voltage, greater than 3.6 volts, relative to the reference electrode (Li/Li.sup.+), smoother voltage declines during discharge of the cell, higher charge capacity, and the replacement of lithium metal anodes with intercalation anodes. In the later case, the cell voltage is the difference in electrochemical potential of lithium within the two intercalating electrodes. This electrode composition has been called the "rocking chair" battery because lithium ions move back and forth between the intercalation compounds during charge/discharge cycles. Potential drawbacks to rocking chair cells include lower output voltage and energy density compared to lithium metal cells. The use of compounds which reversibly intercalate lithium at higher voltage, such as, LiNiO.sub.2, LiCoO.sub.2 and LiMn.sub.2 O.sub.4 may ameliorate these drawbacks. Recently, several manufacturers have expressed their interest in developing batteries based on the use of one of the higher voltage intercalation materials in cathodes and a form of carbon as the intercalation anode.
J. M. Tarascon et al., Electrochim. Acta 38 (1993) 1; J. Electrochem. Soc. 138 (1991) 2864; 139 (1992) 937; 138 (1991) 2859; U.S. Pat. Nos. 5,135,732 and 5,196,279 review the method of making and the use of Li.sub.x Mn.sub.2 O.sub.4 (0&lt;X.ltoreq.2) intercalation electrodes in cells containing liquid electrolytes and lithium metal or carbon anodes, e.g. Li.sub.x C.sub.6 (0.ltoreq.X.ltoreq.1). The disclosure of each of the foregoing references is incorporated herein in its entirety.
The use of intercalation cathodes or anodes composed of mixed oxides and other materials has led to conflicting claims in the technical literature of lithium liquid electrolyte cells. U.S. Pat. No. 4,310,609 discloses a composite cathode material comprising at least one metal oxide incorporated in the matrix of a host metal oxide for use in a non-aqueous liquid electrochemical cell with a lithium metal anode. The metal oxides are thermally treated in the mixed state. The cathode material of this disclosure can be constructed by the chemical addition, reaction, or otherwise intimate contact of various metal oxides or metal elements during thermal treatment in mixed states. Alternatively, the metal oxide may be the product of the thermal treatment of a single metal oxide. U.S. Pat. No. 4,770,960 reports a lithium liquid electrolyte cell using a cathode active material which is the complex oxide LiCo.sub.1-x Ni.sub.x O.sub.2 made from calcined mixtures of LiCO.sub.3, CoCO.sub.3 and NiCO.sub.3. The cell voltage was reported to be less than 2.0 volts vs. a Li/Li.sup.+ anode. Furthermore, the discharge capacity decreased with the increase in nickel content for x&gt;0.27. The recommended cathode active materials were those having 0&lt;x.ltoreq.0.27. U.S. Pat. No. 5,053,297 discloses cathode active materials which contain as a primary active material a first lithium compound having an electrochemical potential which is more noble than the electrochemical potential of the current collector, and an auxiliary active material which is a second lithium compound having an electrochemical potential which is more base than the electrochemical potential of the current collector. Examples include physical mixtures of LiNiO.sub.2 and LiCoO.sub.2, as well as, chemical mixtures i.e. LiNi.sub.0.95 Co.sub.0.05 O.sub.2, for use in lithium liquid electrochemical cells. The electrolyte may alternatively be a gel electrolyte. The addition of auxiliary active material decreases the battery capacity. The preferred anode is a carbon material. European patent application 91119471.0 (Publication 0486950Al) discloses a liquid electrolyte secondary lithium cell having an intercalation carbon anode and a cathode which comprises a lithium-containing metal complex oxide of the formula Li.sub.x MO.sub.2, wherein x is 0.5.ltoreq.x.ltoreq.1 and M is selected from the group Co, Ni and Mn. Examples of the metal complex oxides include LiCoO.sub.2, LiNiO.sub.2, LiNi.sub.y Co.sub.1-y O.sub.2, (0&lt;y&lt;1), LiMn.sub.2 O.sub.4 and mixtures thereof. The cathode active material is ordinarily used in combination with a conductive agent such as graphite and a binder therefor such as polyvinylidene fluoride. The average discharge voltage of the cell is about 3.6 volts. Application PCT/US92/00348 (Publication WO92/13367) describes a secondary lithium cell with liquid electrolyte and intercalation electrodes consisting of Li.sub.x Mn.sub.2 O.sub.4 (cathode) 1.ltoreq.x.ltoreq.2, and Li.sub.x C(carbon anode) 0.ltoreq.x.ltoreq.1. U.S. Pat. Nos. 5,069,683 and 5,028,500 report the use of intercalation carbon electrodes in liquid electrolyte secondary lithium electrochemical cells characterized by the "degree of graphitization" of the carbon: essentially, a mixed phase carbon electrode consisting of a mixture selected from carbon black, graphite and coke. Cathode active materials suggested for this cell are LiNiO.sub.2, LiCoO.sub.2, and complex oxides of the same. The disclosures of each of the foregoing references is incorporated herein in its entirety.
It is characteristic of the higher capacity intercalation compounds used as active cathode materials that each compound accepts lithium ions at a series of unique voltages. Typically, the discharge curves include a series of inflections. LiMn.sub.2 O.sub.4 produces voltage plateaus at 4.1 volts, 3.9 volts and 2.9 volts, (versus Li/Li.sup.+) on cell discharge. LiCoO.sub.2 produces a voltage plateau at 3.7 volts. It would be advantageous if a continuous and smooth voltage profile over a relatively large voltage range could be obtained for the solid lithium cell. Furthermore, it would be highly desirable to tailor the voltage profile to the requirements of the cell and its use.
During charging of an electrochemical lithium cell containing a carbon anode, the lithium ions are intercalated from the cathode and intercalated into the carbon anode. However, 20% of the lithium ions are irreversibly intercalated into the carbon anode. It would be desirable to use a cathode material of higher lithium capacity in conjunction with carbon anodes.